An optical tomographic imaging apparatus has been widely used recently as a medical imaging apparatus. Particularly, an optical tomographic imaging apparatus by optical coherence tomography (hereinafter, called “OCT”) using multi-wavelength light interference can obtain a tomographic image of an eye ground (fundus oculi) up to a depth of several millimeters with depth resolution on the order of microns. Therefore, the optical tomographic imaging apparatus has had a growing importance as a diagnostic tool for providing important information which is unable to be obtained by a conventional scanning laser ophthalmoscope (SLO).
OCT in various forms was described in detail in Non-Patent Document 1 (“Optical Coherence Tomography” by M. Brezinski, Wily, London, 2006). A TD-OCT (Time domain OCT) combining a wideband light source with a Michelson interferometer is configured so that it scans a delay of a reference arm to measure light interfering with backscattered light of the signal arm, thus obtaining information on depth resolution.
However, in such TD-OCT, it is difficult to take an image at a high speed, and OCT in a form as described below is known as a method for taking an OCT image at a higher speed. For example, a known OCT for obtaining interferogram with a spectroscope using a wideband light source includes an SD-OCT (Spectral domain OCT). Further, an SS-OCT (Spectrally swept OCT) is known as OCT for obtaining interferogram in chronological order using a frequency scanning light source.
Then, in the SS-OCT described above, the frequency scanning rate of the frequency scanning light source is an important factor for improving the frame rate and the volume rate. The frequency scanning light source is set in Fourier domain mode locking and buffered by a fiber ring, thereby achieving a scan rate of about 200 kHz (A scan rate), Non-Patent Document 2 (“Opt. Exp.” by R. Huber, et al., Vol. 14, pp. 3225, 2006) disclosed that by using this light source, scanning can be performed at a frame rate of about 900 Hz and a volume rate of 3.5 Hz.
Meanwhile, Non-Patent Document 3 (“Opt. Exp.” by S. Yamashita, et al., Vol. 14, pp. 9399, 2006) disclosed that in recent days, OCT uses a wavelength-tunable laser by a dispersion tuning method that has been developed recently in the field of communication technologies. This dispersion tuning method uses the principle that dispersion in a resonator can be utilized to change a wavelength in response to change in mode-locked frequency. According to this principle, a mechanical drive member is not required to scan wavelength at a high speed, which can allow the wavelength to be scanned at a high speed. Further, Patent Document 1 (Japanese Patent Application Laid-Open No. 08-222790) proposed a laser apparatus using such dispersion tuning method and adapted to be capable of controlling oscillation wavelength. This laser apparatus includes a positive dispersion region, a negative dispersion region and two modulators in a ring resonator, and is adapted to make the amount of wavelength dispersion approximately zero and generate a single-wavelength light pulse in the ring resonator.